In a semiconductor device manufacturing sequence, there is a film forming process for forming a nitride film such as a silicon nitride film (SiN film) or the like as an insulating film on a semiconductor wafer represented by a silicon wafer. A chemical vapor deposition (CVD) method is widely used for such a SiN film forming process.
On the other hand, in recent years, along with the progress of miniaturization and high integration of semiconductor devices, from the viewpoint of improvement in characteristics, a SiN film is formed by sequentially supplying gases as in an atomic layer deposition (ALD) method capable of forming a film of good quality at a lower temperature than the film formation by the conventional CVD method. In the case of forming the SiN film by the ALD method, for example, a SiN film having a predetermined film thickness is formed by causing a Si raw material to be adsorbed onto a substrate to be processed, subsequently supplying a nitriding gas so as to react with the Si raw material, forming SiN at an atomic layer or molecular layer level, and repeating the above steps a predetermined number of times.
As a method of forming a SiN film by an ALD method, there has been proposed a technique in which a dichlorosilane (DCS; SiH2Cl2) gas, which is a Si raw material gas, and an ammonia (NH3) gas, which is a nitriding gas, are alternately supplied, and at the time of supplying the NH3, high frequency power is applied so as to generate plasma to promote a nitriding reaction.
Since an apparatus configuration becomes complicated in the case of using plasma, it has been studied to form, as in a thermal ALD method or the like, a SiN film by a plasma-less sequential gas supply.
However, when a SiN film is formed by a thermal ALD method, an incubation time during which film growth does not proceed despite alternately supplying a Si raw material gas and a nitriding gas is generated at an initial stage of film formation. When the incubation time is present, the film formation varies two-dimensionally at the initial stage of film formation. Thus, the film has an island-like distribution, the surface roughness at the time of film formation becomes larger, and weak film portions occur locally. In the case where a DCS gas is used as a Si raw material, the incubation time varies depending on a base film. When an extremely thin film is formed on different base films, there may be generated a state in which a film is formed on one base film but is not formed on the other base film.